As described in U.S. Pat. No. 4,989,202, an ISDN (integrated services digital network) transmission path between the transmission line and a network has an interface. It is also known (see "Siemens KMT Report," no. 14, order no. E 80001-V331-W54, pages 6 and 7) that measurements can be performed on this interface and an appropriate measuring device can be provided for this purpose. For example, frequency-dependent measurements, functional tests and even jitter measurements can be performed with this measuring device. An ISDN data transmission line is a special form of a transmission line with full-duplex transmission in the common carrier duplex process.
According to European Patent Application No. 9,592 A1, a differential unit in a transmission and receiving device connected to a duplex line is provided for receiving signals. Two signals are sent to this differential unit, namely the transmission signal of its own transmission and receiving device and the complete signal that is transmitted on the duplex line and which consists of its own transmission signal plus a signal to be received from another transmission device. The task of the differential unit is to subtract its own transmission signal from the composite signal of the duplex line and in this way to make it possible to receive only the signal of the other transmission device. Thus, cross-talk from its own transmission signal to the received signal is suppressed.